add comment to singletonWarnDoubleRegistrationAndAbort
add comment to singletonWarnDoubleRegistrationAndAbort

Summary: Would help make these types of errors easier to debug

Reviewed By: yfeldblum, ot

Differential Revision: D10857148

fbshipit-source-id: fbb1defbde2f3695e91da92ca502c0a0aed4c7ec

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly